1. Field of the Invention
This invention relates to a method and apparatus of superimposing objects onto a scene of a motion picture.
2. Description of the Prior Art
In motion pictures and in television, it is frequently impractical or impossible to film objects or actors in certain locations or background scenes. For example, if a particular scene in a motion picture required an actor to pass in front of a building that no longer exists or is inaccessible, film makers will superimpose the actor in a scene of the building taken at a separate time or in a scene of a model of the building.
One of the oldest ways to superimpose the actor or object into a scene was by projecting the scene onto a screen behind the actor and photographing the actor in front of the projected image. One projection technique utilized a front projector, which directs light through a half-silvered mirror along the optical axis of the camera to a retro-reflective screen and back to the camera. Because a retro-reflective screen reflects much more of the projected light to the camera than the foreground object reflects, almost none of the background scene appears on the filmed image. These methods are most inconvenient and requires all stock footage to be filmed first. Much is wasted and if the background is to be changed later, the actors must re-shoot the scene with the new background. More recently, other processes have been used.
One process is called the blue-screen process. Essentially the desired foreground object such as an actor is positioned in front of a monochromatic screen. The screen is usually blue, translucent plastic that is illuminated by rear projection, or it is a blue wall illuminated in front. The scene is then filmed, the resultant print is then processed, and the blue background is filtered out. The method uses much film, is time consuming and is labor intensive, having many steps in order to yield a composite scene.
The "sodium-yellow" method, named for the sodium vapor lamp that radiates light in a narrow range of wavelengths, uses yellow light in this narrow band. The actor is illuminated by two sources of light. The first source is general illumination at desired angles to illuminate the foreground object properly. The sources are filtered to eliminate any light in the narrow, yellow wavelength. This lighting is accomplished using well-known lighting techniques, which normally employs two or more separate light sources for shadow and highlight control. The second source of light radiates sodium yellow light rear projected through a screen behind the foreground objects.
The general light reflects from the foreground object to a camera lens which directs the light to a beam splitter. The lens also directs the yellow light from the screen is also directed to the beam splitter. The foreground object blocks the rear screen. The beam splitter is a specially designed precision prism that is designed to reflect all light in the narrow band to one film plane and pass all other light to the other film plane.
Along the first path, the light is focused and exposes a first film negative. Lighting levels and camera exposure are set so that the yellow light exposes that portion of the film negative that receives light from the rear projection screen. Because of the filters on the first, general source, the foreground object receives and reflects no light from the first source in the narrow band. The light from the first, general source that illuminates the foreground object passes through the beam splitter and exposes the image of the foreground object on a second negative normally except that the light in the narrow wavelengths is filtered out.
Therefore, two negatives are formed. The first has a barely exposed, almost clear area for the foreground object and an exposed, black area for the screen. In the second negative, the screen is essentially clear, but the foreground object is normally exposed. When positives are made from each negative, the first exposure has a black silhouette with a transparent outline where the foreground screen was.
It is very difficult, however, to construct a beam splitting prism properly. Extremely minor imperfections in the dielectric coatings of the prism result in significant degrading of the mattes so that the system does not work. As a result, there are only a small number of sodium-yellow systems in existence.
It is also been proposed to use light outside of the visible spectrum, i.e. infrared or ultraviolet light. Both have focusing problems because of their shorter or longer wavelengths with respect to the average focusing of light of the visible spectrum. Ultraviolet light creates some danger to actors, so shooting periods must be short. Typical foreground objects such as people also radiate some infrared light. Therefore, using infrared or ultraviolet light has proved unsatisfactory.